In the fabrication of pipe made, for example, from relatively thick plates, for instance by the UOE method, the press method or the three-roll bending method, a round elongated slitted tube is formed which is then welded closed along a continuous longitudinal weld seam. In the welding to form the pipe, the material from which the pipe is formed, generally thick sheet or plate steel, may be subjected to thermal stress. As a result of the welding process the pipe is distorted during fabrication and may not be straight, i.e. can have a banana shape. As a consequence, the pipe may have to be subjected to a straightening operation.
For the straightening of such pipe devices which engage the pipe from the exterior have been used by and large almost exclusively. The apparatus for this purpose may apply straightening rollers or saddles to the outer surface of the pipe with sufficient force to apply bending moments to the pipe and straighten the latter. The external bending devices may be used in conjunction with an expander head inserted into the pipe.
The devices which engage the pipe from the exterior require a significant superstructure adapted to take up the bending stresses that must be applied and which can be costly and occupy significant space.
Another drawback of conventional pipe straightening machines is that the straightening effect usually is effective only in one direction, for example, in a vertical direction which may require transmissions of the bending force over a long distance. The supporting structure in such cases may have a relatively high degree of elasticity and thus take up a significant part of the bending force by distortion. The result may not be satisfactory straightening.
Since pipes are fabricated with greater and greater wall thicknesses and materials of increasing strength, conventional pipe straightening equipment may have reached limits in straightening capacity and may not be able to straighten pipes manufactured more recently in an efficient and effective manner.